Polymeric microspheres

ABSTRACT

The invention features core-shell microsphere compositions, hollow polymeric microspheres, and methods for making the microspheres. The microspheres are characterized as having a polymeric shell with consistent shell thickness.

This application is a divisional application of, and claims priority to,U.S. patent application Ser. No. 10/823,367, filed Apr. 12, 2004, whichis a continuation of U.S. patent application Ser. No. 10/033,389, filedOct. 25, 2001, now U.S. Pat. No. 6,720,007, which claims priority toU.S. provisional application 60/243,104, filed Oct. 25, 2000. The entirecontents of each of the aforementioned applications is incorporatedherein by reference.

STATEMENT AS TO FEDERALLY-SPONSORED RESEARCH

This invention was made with government support under grant numberGM48142 awarded by the National Institutes of Health, grant numberDE-AC05-000R22725 awarded by the Department of Energy, andDAAK60-97K-9502 awarded by the U.S. Army. The U.S. government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

The invention relates to microsphere particles.

Hollow microsphere particles have a wide variety industrial andbiomedical uses. However, the formation of uniform and regular shellstructures, as well as control over the shell thickness, is difficult toachieve using present methods, thereby restricting the uses of suchparticles.

SUMMARY OF THE INVENTION

The invention features hollow microspheres and core-shell microspherecompositions with consistent shell thickness using methods, which allowcontrolled formation of a polymeric shell. The thickness of thepolymeric shell preferably varies less than 10%, more preferably lessthan 5%, more preferably, less than 1%, and most preferably less than0.5%. The variability in the thickness of the polymeric shell isdetermined by measuring the thickness at two or more points on themicrosphere and calculating % divergence.

Shell thickness is controlled by the length of polymerization and isvaried to provide microspheres for divergent applications such as drugdelivery or synthetic pigment preparation. Duration of thepolymerization step is directly proportionate to the length of thepolymer chains, and thus, shell thickness. Shell thickness is in therange of 100-1000 nm. In preferred embodiments, the shell thickness isin the range of 150-250 nm. Alternatively, the shell thickness is in therange of 350-450 nm or in the range of 550-650 nm. Preferably, themicrosphere is substantially devoid of silica. For example, themicrosphere contains less than 10%, more preferably less than 5%, morepreferably, less than 1% silica by weight.

The microspheres contain pores. A pore is a void in the polymeric shellthrough which a composition may gain access to the hollow portion of themicrosphere. The microspheres have a certain porosity, and the porosityis varied depending on the size and composition of the substrate used tomake the sphere. Pore size is varied depending on the size and nature ofthe composition to be loaded into the hollow center of the sphere aswell as by changing the amount of crosslinking agent added duringpolymerization. For example, the addition of increasing amounts of acrosslinking agent produces microspheres with decreasing pore size. Poresize is also affected by the addition of a foaming agent, i.e., additionof a foaming agent during production of the shell increases pore size.For example, a pore has a diameter in the range of 10-500 nm.

Microspheres are useful as synthetic pigments, drug delivery vehicles,and protecting agents. For example, the microspheres are used in placeof titanium dioxide, i.e., as a synthetic pigment, because an emptymicrosphere in solution appears white. Organic dyes are encapsulated ina hollow microsphere to produce a synthetic pigment of a desired color.Empty or dye-encapsulated microspheres have several advantages overstandard titanium dioxide-based paints or dyes, e.g., improved colorclarity or trueness.

The microsphere is also useful as protecting agent. For example, alight-sensitive compound (e.g., a photo-bleachable dye) is loaded into ahollow microsphere to protect its degradation from exposure to light orchemicals prior to use. The compound is released from protection bydisrupting the microsphere, e.g., by crushing the sphere or contactingthe sphere with a solvent.

In addition to industrial applications, microspheres are used asdelivery vehicles for therapeutic agents such as polypeptides,antibodies, enzymes, small molecule drugs, or nucleic acids.

The nature of the polymeric shell is varied to accommodate various usesof the hollow microspheres. The microsphere shell typically containsstyrene, methacrylate, or any polymer with a high glass-transitiontemperature (T_(g)). The shell contains a polymer resulting from thepolymerization of one or more monomers selected from the groupconsisting of acrylonitrile, styrene, benzyl methacrylate, phenylmethacrylate, ethyl methacrylate, divinyl benzene, 2-hydroxyethylmethacrylate, cyclohexyl methacrylate, p-methyl styrene, acrylamide,methacrylamide, methacrylonitrile, hydroxypropyl methacrylate, methoxystyrene, N-acrylylglycinamide, and N-methacrylylglycinamide.Alternatively, the shell contains a copolymer (random or block) selectedfrom the group consisting of styrene-PMMA, benzyl methacrylate-PMMA,styrene-PHEMA, styrene-PEMA, styrene-methacrylate, andstyrene-butylacrylate. The strength and durability of the polymericshell is increased by crosslinking polymer chains.

The invention also includes methods of making hollow microspheres byproviding a substrate containing a plurality of hydroxyl groups andattaching an initiator agent to the hydroxyl groups to form attachedinitiator agents. Any solid substrate, which is characterized ascontaining hydroxyl groups on its surface and is dissolvable (followingpolymerization of the shell) is suitable. For example, the substrate issilica, alumina, mica, or a clay composition. Alternatively, thesubstrate is a crystal, which has been coated with a silica. Theinitiator agents react with a polymerizable unit under polymerizationconditions to form a polymer shell over the substrate. Thepolymerization is confined to a surface of the substrate. A polymerchain is initiated at the initiator agent and is extended away from thesubstrate during polymerization. To remove the substrate from thepolymeric shell (to yield a hollow microsphere), the substrate iscontacted with an etching agent for a time sufficient to allow forelimination of the substrate from the polymeric shell. An etching agentis a composition which removes a solid substrate from the center of apolymer-coated substrate, leaving a polymeric shell. Preferably, atleast 85% of the substrate, more preferably 95%, more preferably 99%,and most preferably 100% of the substrate is removed from the core ofthe sphere. Etching agents include bases or acids, e.g., hydrochloricacid (HCl), hydrogen fluoride (HF), sulfuric acid (H₂SO₄), sodiumhydroxide (NaOH), potassium hydroxide (KOH). Alternatively, thesubstrate is metal, and the etching agent is an oxidizing or reducingagent. For example, a silica substrate or mica is removed by etchingwith HF, and an alumina or clay substrate is removed by etching withKOH. Optionally, the method includes a step of exposing the polymershell to a crosslinking agent.

The polymerizable unit is a monomer selected from the group consistingof acrylonitrile, styrene, benzyl methacrylate, phenyl methacrylate,ethyl methacrylate, divinyl benzene, 2-hydroxyethyl methacrylate,cyclohexyl methacrylate, p-methyl styrene, acrylamide, methacrylamide,methacrylonitrile, hydroxypropyl methacrylate, methoxy styrene,N-acrylylglycinamide, and N-methacrylylglycinamide or a co-polymerselected from the group consisting of styrene-PMMA, benzylmethacrylate-PMMA, styrene-PHEMA, styrene-PEMA, styrene-methacrylate,and styrene-butylacrylate. Thickness of the developing polymeric shellis controlled by the length of polymerization.

The invention also includes a core-shell composition. A core-shellcomposition is a composition, which contains at least two structuraldomains. For example, the core domain is encased in the shell domain,and the shell domain is characterized as having different physical andchemical properties than the core. The core portion contains a firstcompound, and the shell contains a second compound (which is not presentin the core portion). The core and shell differ by the presence orabsence of at least one compound. A method for preparing a core-shellcomposite includes the following steps: providing a microspheresubstrate; contacting the microsphere substrate with a polymernanosphere to yield a colloidal assembly; and heating the assembly toyield a core-shell composite.

An alternative method for preparing a hollow microsphere includes thefollowing steps: providing a microsphere substrate; contacting themicrosphere substrate with a polymer nanosphere to yield a colloidalassembly; heating the assembly to yield a core-shell composite; andexposing the composite to an etching agent for a time sufficient toallow for removal of a core composition, e.g., silica, from the shellpolymer composition to form a hollow microsphere.

A colloidal assemby is an organized structure of two or more particletypes. For example, the assembly is organized such that the nanospheresare assembled onto the surface of a microsphere. Preferably, themicrosphere is 1-100 μm in diameter; more preferably, the microsphere isless than 75 μm in diameter; more preferably, the microsphere is lessthan 50 μm in diameter; and even more preferably the microsphere is lessthan 25 tm in diameter. For example, the microsphere is 3-10 μm indiameter. The nanosphere is 1-1000 nm in diameter. Preferably, thenanosphere is less than 500 nm; more preferably, the nanosphere is lessthan 250 nm. For example, the nanosphere is 100-200 nm in diameter.

The nanospheres and/or microspheres are optionally modified to contain areactive substituent. Preferably, the microsphere and nanosphere containdifferent substituents, which associate, bind, or react with oneanother. For example, the nanosphere contains an amine-modified polymer,e.g., an amine-modified polystyrene (PS), and the microsphere comprisesan aldehyde-modified composition, e.g., glutaraldehyde-activated silica.The microsphere substrate contains silica, alumina, mica, or clay. Inanother example, the nanosphere contains avidin and the microspherecontains biotin, or the nanosphere contains biotin and the microspherecontains avidin. The nanosphere may contain one type of polymer or amixture of polymers. For example, the nanosphere contains PS, PMMA, orboth. The microspheres are optionally contacted with a mixture ofdifferent nanospheres, e.g., a mixture of PS nanospheres and PMMAnanospheres, to yield a composite polymer shell. The ratio of differentpolymer nanospheres is varied to achieve a desired effect, e.g.,strength or porosity. For example, the ratio of PS: PMMA is 50:50,100:1, 10:1, 5:1, or 2:1.

The colloidal assembly is heated to a temperature greater than the T. ofthe polymer nanosphere to melt the polymer nanospheres. The polymerflows over the microsphere surface resulting in an essentially uniformcoating, i.e., the thickness of the polymer shell varies less than 10%over its entire surface. For example, the colloidal assembly is heatedto at least 100° C. To melt PS and/or PMMA nanospheres, the colloidalassembly is heated to 170-180° C.

Other features and advantages of the invention will be apparent from thefollowing description of the preferred embodiments thereof, and from theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are scanning electron micrographs of hollowmicrospheres. FIG. 1A is a micrograph of silanized silica microspheres,and FIG. 1B is a micrograph of the same microspheres after coating withpoly(benzyl methacrylate) by controlled/living radical polymerizationfor 14 h.

FIGS. 2A and 2B are scanning electron micrographs of the polymermicrospheres after etching with HF. FIG. 2A is a scanning electronmicrograph of etched microspheres, and FIG. 2B shows the microspheresdispersed in water to allow visualization of individual particles.

FIG. 3 is a scanning electron micrograph of the hollow polymericmicrospheres obtained by crushing the hollow spheres by applyingphysical pressure after freezing in liquid nitrogen. Both broken andintact polymer spheres are seen.

FIGS. 4A-C are scanning electron micrographs of broken microspheresisolated after different polymerization times: FIG. 4A, 3.5 hpolymerization; FIG. 4B, 6.5 h polymerization; and FIG. 4C, 14 hpolymerization.

FIG. 5 is a linear plot of a FTIR spectrum. Plot (a) was obtained usingpure silica particles; plot (b) was obtained using hybrid poly(benzylmethacrylate)/silica particles; plot (c) was obtained using hollow PBzMAmicrospheres after silica core etching; and plot (d) was obtained usingpure poly(benzyl methacrylate).

FIGS. 6A-B are scanning electron micrographs of hollow shellcross-linked PBzMA microspheres. FIG. 6A shows intact hollowmicrospheres, and FIG. 6B shows broken hollow microspheres.

FIG. 7 is a linear graph of a FITR spectrum of the shell cross-linkedhollow poly(benzyl methacrylate) microspheres.

FIG. 8 is a graph of a tapping mode AFM scan of the surface of hollowPBzMA microspheres.

FIG. 9 is a diagram showing a polymerization step for coating silicaparticles.

FIG. 10 is a diagram showing a method for making hollow microspheres.

FIG. 11 is a line graph showing release of fluoroscein from hollowmicrospheres (dashed line) compared to release from coated beads overtime.

FIG. 12 is a diagram showing a scheme for assembling composite materialsvia both glutaraldehyde chemistry and biospecific interactions. The topsection illustrates assembly starting with amine-labeled silica.Glutaraldehyde treatment followed by reaction with amine-modifiedpolystyrene nanospheres results in a silica-polymer composite that canbe heated at 170-180° C. to melt the polystyrene. A core-shell materialcomposed of a silica core and polystyrene shell is produced. The bottomsection illustrates assembly of biotin-labeled polystyrene nanoparticlesonto avidin-coated silica microspheres.

FIGS. 13A-D are scanning electron micrographs of particle assemblies,FIG. 13A shows 3.0 μm mean diameter glutaraldehyde coated silicaassembled with 100 nm mean diameter amino-functionalized polystyreneparticles. FIG. 13B shows 5.0 μm mean diameter glutaraldehyde coatedsilica assembled with 200 nm mean diameter amino polystyrenenanoparticles. FIG. 13C shows 5.0 μm mean diameter avidin-coated silicaassembled with 100 nm mean diameter biotin-coated polystyrenenanoparticles. FIG. 13D shows polystyrene film coated silica particles,which result from heating microspheres identical to those shown in FIG.13A to 170-180° C. in ethylene glycol.

FIG. 14 is a tapping mode scanning force microscopy (SFM) scan of thesurface of the composite produced when 200 nm PS nanospheres areassembled on 5 μm diameter silica.

FIG. 15 is a scanning electron micrograph of a non-specific bindingcontrol. 100 nm amine-modified nanospheres were mixed with amine-labeledsilica microspheres under conditions identical to those in the assemblyprocess.

FIG. 16 is a transmission electron micrograph (TEM) of hollowmicrospheres after chemical etching of core-shell materials produced byassembly/melting of 200 nm PS nanospheres with 5 μm diameter silicamicrospheres.

FIGS. 17A-C are line graphs showing data from FTIR spectroscopy of purepolystyrene (FIG. 17A), melted PS-silica composite (FIG. 17B) andaldehyde-activated silica (FIG. 17C). FIG. 17A shows a comparison of thespectra in the range of 650 cm-1 to 780 cm-1. Bands at 750 cm-1 and 697cm-1 correspond to polystyrene benzene C—H stetching and ring bendingvibrations.

FIG. 18 is a line graph showing a comparison of FTIR spectra of purepolystyrene and PS-silica composite. The spectra shown are in the rangeof 2850 cm⁻¹ to 2950 cm⁻¹. Peaks in this wavenumber range correspond tothe aliphatic C—H stretching of polystyrene.

FIG. 19 is a line graph showing FTIR spectra of different core-shellcomposites. The top spectrum (PS/silica composite) has peakscorresponding to both silica and PS, while the middle spectrum(PS/PMMA/silica composite) has an additional peak corresponding to thecarbonyl of the PMMA polymer. The bottom spectrum is from plain silicamicrospheres.

DETAILED DESCRIPTION

Hollow polymer microspheres were prepared by coating silica microspheretemplates with poly(benzyl methacrylate) using surface initiatedcontrolled/living radical polymerization and subsequently removing thecore by chemical etching. Shell thickness was controlled by varying thepolymerization time. Scanning electron microscopy was used tocharacterize the products and demonstrate that the polymer microsphereswere hollow. FTIR spectroscopy showed that the silica cores werecompletely removed by etching.

Surface Confined Living Radical Polymerization: A New Method forPreparing Hollow Polymer Microspheres on Silica Templates.

Uniform hollow polymeric microspheres were made by using surfaceconfined living radical polymerization. Using the silica microsphere asa sacrificial core, hollow microspheres were produced following coredissolution. First, a controlled/living polymerization was conductedusing an initiator attached to the surface of silica microparticles toinitiate atom transfer radical polymerization (ATRP). This procedureyielded core-shell microparticles with a silica core and an outer layerof covalently attached, well-defined, uniform thickness polymers. Thesilica cores were subsequently dissolved, resulting in hollow polymericmicrospheres. Silica microspheres were coated with polymer in two steps(FIG. 9). For example, first a benzyl chloride monolayer was prepared bysilanization of silica microspheres. In the second step,surface-modified silica microspheres were heated in presence of copperhalide, complexing agent (dipyridyl) and benzyl methacrylate monomer inxylene at high temperature to prepare uniformly coated microspheres.Polymer coated silica microspheres were then immersed in aqueous HFsolution to yield uniform hollow poly(benzyl methacrylate) (PBzMA)microspheres. In order to confirm that the microspheres are hollow, themicrospheres were first frozen in liquid nitrogen and then crushedbetween two glass plates. FIG. 3 shows a SEM image of intact and brokenhollow polymer microspheres.

The process by which hollow polymeric microspheres are made differssignificantly from methods known in the art. The method presented hereinfor preparing uniform hollow polymeric beads utilizes surface confinedliving radical polymerization technique on silica templates.

Earlier processes for making hollow polymer latex particles include onedeveloped by Rohm and Haas. This earlier process involved makingstructured particles with a carboxylated core polymer and one or moreouter shells. The ionization of the carboxylated core with base expandsthe core by osmotic swelling to produce hollow polymer particles.Another method involved an emulsion polymerization of styrene containinga small amount of vinyl carboxylic acid in the presence of ahydrocarbon, surfactant, and a water miscible alcohol. These processesare complex and involve several steps employing different chemistries.Other methods involve the synthesis of hollow nanoscopic polypyrroleparticles to employ gold nanoparticles as a template from which to growthe polymer shell, followed by dissolution of the template. The methodsof the invention offer several advantages over earlier know methods inthat the present process yields microspheres with relative uniformity ofshell thickness. Another advantage is that the shell thickness isreliably and consistently controllable. In contrast, formation ofuniform and regular shell structures surrounding the particles, as wellas control over the shell thickness, has been difficult to achieve usingthe earlier methods, because polymerization is not restricted to thesurface of the templates.

The surface confined living radical polymerization method presentedherein is simple, flexible and enables control over the shell thicknessand composition by adjusting polymerization time and monomerconcentration. Unwanted solution phase polymerization is also preventedusing this method. This method is applicable for preparing variety ofhollow polymer microspheres. This approach may allow for the fabricationof different shapes of hollow polymeric materials produced from avariety of templates.

A wide range of monomers are used to make the shell of the hollowmicrosphere. Examples of monomers, which are compatible with the livingpolymerization procedure, are listed in Table 1.

TABLE 1 Monomers for shell construction Acrylonitrile Styrene Benzylmethacrylate Phenyl methacrylate Ethyl methacrylate Divinyl benzene2-Hydroxyethyl methacrylate Cyclohexyl methacrylate p-Methyl styreneAcrylamide Methacrylamide Methacrylonitrile Hydroxypropyl methacrylateMethoy styrene N-Acrylylglycinamide N-Methacrylylglycinamide

Co-polymers may also be produced. A list of suitable co-polymers isprovided in Table 2. Monomer designations are abbreviated as follows:PMMA, Poly(methyl methacrylate); PHEMA, Poly(hydroxyethyl methacrylate);PEMA, Poly(ethyl methacrylate).

TABLE 2 Co-polymers Styrene-PMMA Benzyl methacrylate-PMMA Styrene-PHEMAStyrene-PEMA Styrene-Methacrylate Styrene-Butylacrylate

As is described above, a silica particle serves as a scaffold upon whichthe shell is built by polymerization. An initiator is attached to thesurface of silica microparticles to initiate atom transfer radicalpolymerization (ATRP). Examples of Living Radical Initiators (which areimmobilized on microsphere surfaces by silanization or some other methodprior to living radical polymerization) include those listed in Table 3.

TABLE 3 Living Radical Initiators Phenyl ethyl chloride Phenyl ethylbromide Phenyl sulfonyl chloride 2-Bromoethylisobutyrate

Uses for Hollow Microspheres

Hollow polymer microspheres are a class of materials which haveapplication in the fields of medicine and materials science. Forexample, the microspheres are used for product encapsulation forcontrolled release of drugs and dyes, protection of light sensitivecompounds, enzyme encapsulations, and adhesives.

Hollow microspheres are made in a wide range of sizes (inner/outerdiameter) to suit a particular application. Pore size may also be variedto order. Pore size is controlled by varying the amount of crosslinkerin the polymerization mixture. Generally, the pore size of the hollowmicrosphere is slightly larger than the pore size of the template due tothe etching process. A larger pore size is suitable for adhesiveapplications whereas a smaller pore size suits encapsulation of enzymesor other therapeutic agents. Spheres with a pore size of 100 nm to 500nm (e.g., a sphere, which has a diameter of at least 1 micron sphere inwhich the diameter of the pore is greater than about 10% of the diameterof the sphere) are useful in the formulation of adhesives.

Synthetic Pigments

Hollow polymer nanospheres and microspheres are useful in the industrialproduction of paints and pigments. The microspheres are added toconvention paint formulations as extenders. Hollow polymer microspheresand nanospheres (e.g., 300-500 nm outer diameter) are used in theproduction of synthetic pigments. Hollow polymer particles with outerdiameters in the range of 1-5 microns in diameter have been producedusing the methods described herein. These particles are useful in theproduction of synthetic pigments as well. Nanometer-scale hollow spheresare produced by performing living polymerization with 300-500 nmdiameter silica templates, followed by silica etching in hydrofluoricacid.

The method described herein for preparation of hollow polymermicrospheres may have several advantages over current methods forpreparing synthetic pigments. Since synthetic pigments are made fromhollow polymer spheres, the shell thickness and composition must becontrolled in some manner. Shell thickness is correlated with theopacity of the resulting pigment. Template directed livingpolymerization allows the polymer shell thickness to be more accuratelycontrolled than in other known methods. This results in better controlof the opacity of the resulting pigment. Polydispersity of the polymerin the spheres also influences opacity. By using living polymerizationmethods, polydispersity is also under better control and more consistentthan in other methods. Polydispersity also influences the uniformity ofthe hollow microsphere surface.

Material Encapsulation/Drug Delivery

Hollow polymer microspheres also have application in the fields ofmaterials encapsulation and drug delivery. Drugs such as tranilast oribuprofen are encapsulated in polymeric microspheres. The spheres areused to slowly release drug over time in the digestive tract.Biocompatible hydrogels such as polyacrylamide-chitosan are useful forsustained antibiotic release. For example, microspheres with a core sizeof approximately 3 micrometers are used for drug delivery.

Microspheres produced by living polymerization are more advantageous fordrug delivery applications because of the consistency in shell thicknessand porosity. In contrast, the shell thickness of the microspheresproduced by existing technology cannot be controlled during thepolymerization. Being able to control the shell thickness and thereforethe rate of drug release from the microspheres is a significantadvantage of the microspheres of the invention.

Studies on encapsulation and release of test agents, e.g., a dye, fromhollow microspheres were carried out as follows. Crosslinked hollowpoly(benzyl methacrylate) or PBzMA coated silica (approximately 1 mg)beads were soaked with fluorescein in a methanol solution overnight.Excess fluorescein was removed by centrifugation (1000 rpm), followed bya wash with methanol. The dye-loaded beads were immersed in 0.5 mlmethanol. Release of the dye from hollow microspheres was monitored bymeasuring the increase in fluorescence of the surrounding solution as afunction of time. The data shown in FIG. 11 indicate that hollowmicrospheres or beads (dashed line) are effectively loaded with acomposition of interest and that the composition is released from thehollow microspheres into the surrounding environment in a time-dependentmanner. In contrast, the composition is not loaded (and therefore, notreleased) from coated solid beads (solid line) under the sameconditions.

Microspheres produced for delivery of therapeutic products are washedwith water or a physiologically-compatible buffer (e.g.,phosphate-buffered saline) following the etching procedure to remove thesilica template and residual etching agent. The microspheres are thencontacted with a therapeutic agent in solution phase. The microspheresare loaded with the agent by diffusion.

Block co-polymer hollow microspheres may be produced using the livingpolymerization method. The composition of the blocks can be tailored forparticular drug delivery applications.

Protecting Agents

Hollow polymer microspheres are used as protecting agents to stabilizematerials from exposure to light, solvents or other exposures to whichthey may be sensitive. For example, a sensitive composition is loadedinto the microspheres. The composition is protected from light orexposure to other damaging agents until the microsphere is physically orchemically disrupted and the compositions is released from themicrosphere.

Hollow microspheres are also used as coatings. The polymer used to makethe polymeric shell of the hollow microsphere is tailored to theapplication desired. For example, acrylate or methacrylate polymers aresuitable for most coating applications. Such microspheres are useful assunscreen compositions. The microspheres are used alone or incombination with standard sunscreen compositions.

Hollow microsphere coatings are applied to paper or photographs toprotect them from light-mediated aging.

Production of Hollow Microspheres

The process for making uniform hollow polymeric microspheres utilizessurface confined living radical polymerization. Using the silicamicrosphere as a sacrificial core, hollow microspheres are producedfollowing core dissolution. First, a controlled/living polymerization isconducted using an initiator attached to the surface of silicamicroparticles to initiate atom transfer radical polymerization (ATRP).This procedure yields core-shell microparticles with a silica core andan outer layer of covalently attached, well-defined uniform thicknesspoly(benzyl methacrylate) (FIG. 9). The silica cores are subsequentlydissolved, resulting in hollow polymeric microspheres (FIG. 10).Surface-initiated living polymerization is a polymerization process inwhich control of molecular weight is controlled by adjusting the monomerconcentration and termination reactions are substantially eliminated.Polydispersity is thus lowered, enabling fine control of shellthickness, i.e., the shell surfaces are more uniform. Shell thicknessand variations in shell thickness between two or more locations of thesphere are measured using methods known in the art.

In comparison to standard solution or bulk living polymerization,surface-initiated living radical polymerization has several advantages.The growing radicals generated on the surface are not easily terminatedby bimolecular reactions due to limitations of the solid surface onwhich the polymer chains are chemically attached, the low free radicalconcentration and the low mobility. By using a controlled livingpolymerization procedure to covalently attach polymer chains tomicrosphere surfaces, one can control the thickness and uniformity ofthe coated polymer film. Unwanted solution phase polymerization is alsoprevented using this method. An additional benefit is the ability toprepare block copolymers by the sequential activation of the dormantchain end in the presence of different monomers. Although grafting ofpolymers on flat and porous silica surfaces by using living radicalpolymerization (von Werne et al., 1999, J. Am. Chem. Soc. 121:7409-7410)has been described, the procedure for making uniform hollow polymericbeads by using the living radical polymerization technique on a silicamicrosphere template is completely new. The process of von Werne et al.is a method of making a composite film by grafting flat and poroussilica surfaces particles using living radical polymerization.

Rather than producing microspheres, the method of von Werne et al.yields a hexagonally-ordered film with embedded silica nanoparticles. Incontrast, the inventive method yields hollow microspheres with shells ofconsistent thickness.

Reagents

Benzyl methacrylate, ethylene glycol dimethacrylate, 2,2′-dipyridyl(99%) and copper (I) chloride were purchased from Aldrich Chemical Co(Milwaukee, Wis.). Luna porous silica beads (˜3 μm) were purchased fromPhenomenex (Torrance, Calif.) and Bangs silica beads (˜3.1 μm) were fromBangs Laboratories, Inc. (Fishers, Ind.). ((Chloromethyl)-phenylethyl)trimethoxysilane (CTMS) was obtained from Gelest, Inc (Tullytown, Pa.).High performance liquid chromatography (HPLC) grade solvents were usedin both the reaction and washing steps. All the reagents were usedwithout further purification.

Preparation of Poly(benzyl methacrylate)-Coated Silica Microspheres

The reactions for bead coating consisted of two steps, which areschematically shown in FIG. 9. The silica particles were first cleanedwith acetone several times to remove potential impurities. A benzylchloride monolayer was prepared by silanization of silica microspheres.A mixture of 0.9 ml of acetone and 0.1 ml of CTMS were added to 6-7 mgof purified silica microspheres in a 1.5 ml polypropylenemicrocentrifuge tube. The bead suspension was shaken at room temperaturefor 2 h in the dark. After silanization, the silica beads were separatedfrom the suspension by centrifugation, washed with acetone to removeunreacted silane coupling agent and then cured at room temperatureovernight in the dark.

In the second step, the living radical polymerization was performed. A 4ml glass vial was charged with 6-7 mg of the silanized silicamicrospheres and 0.75 ml of dry p-xylene. Dry argon gas was bubbledthrough the mixture for 15 min to remove oxygen from the polymerizationsystem. After the removal of oxygen, 0.0067 g (0.068 mmol) of CuCl,0.0316 g (0.21 mmol) of 2,2′-dipyridyl and 0.75 ml of benzylmethacrylate were added to the same reaction mixture. The vial was thensealed with a high temperature silicone rubber septum and argon wasbubbled through the mixture for another 20 min. to ensure that oxygenwas removed completely. The mixture was sonicated for 1 min toaccelerate dissolution into xylene. The reaction was heated withconstant stirring (with a magnetic stir bar) at 105-110° C. using asilicone oil bath. Polymerization time was varied from 1 to 14 h toproduce polymer shells with different thicknesses. After polymerization,the coated microspheres were separated from the suspension bycentrifugation, and then washed several times bycentrifuging/resuspending in THF and methanol. Cross-linked polymershells were prepared by adding 10% ethylene glycol dimethacrylate (withrespect to benzyl methacrylate monomer) into the above mixture. The restof the procedures were the same as those for linear polymerization.

Procedure for Making Hollow Polymeric Microspheres

The synthesis of hollow polymeric microspheres is schematicallyrepresented in FIG. 10. Briefly, PBzMA coated silica particles werefirst suspended in tetrahydrofuran (THF). The bead suspension wasfiltered through a 0.5 micron pore size Fluopore membrane (MilliporeCorporation, Bedford, Mass.). A thin pellet of coated microspheres wasformed on the top of the membrane. The product was dried in an oven at60° C. for 2 h. A 10% aqueous hydrogen fluoride (HF) solution wasprepared by diluting 50% HF with ultra-pure water. The membranecontaining the pellet was placed in a small polystyrene Petri dish andthen 3.25 ml of 10% HF solution was added to immerse the pellet. Thereaction was allowed to continue for 3 h at room temperature to etch thesilica cores completely. The film was then withdrawn, dipped inultra-pure water which was replaced with fresh water 4-5 times to removeall the unreacted HF. Finally, the pellet was redispersed in water toobtain the individual hollow PBzMA microspheres.

Fourier Transform Infra Red (FTIR) Spectroscopy

FTIR (Nicolet Magna-760, Nicolet Instrument Corporation, Madison, Wis.)spectroscopy was used to identify a polymer on the bead surface and alsoto ensure that silica was removed from the inside of the hollowpolymeric bead. Spectra were obtained at a resolution of 2 cm⁻¹ andaverages of 64-100 spectra/scans (for enhanced signal) were obtained inthe wavenumber range 400˜4000 cm⁻¹. Spectra of the pure silica andpolymer coated silica were recorded from KBr pellets, prepared by mixingthe microspheres with KBr in 1:100(wt/wt) ratio. FTIR spectra for thepure PBzMA and the hollow polymer beads were obtained at roomtemperature by casting a THF solution on KBr pellets. FTIR spectra ofthe shell cross-linked hollow PBzMA microspheres were also measured fromKBr pellets, prepared by the same procedure described above.

Scanning Electron Microscopy (SEM)

SEM was performed using a JEOL SM 840 scanning electron microscope(JEOL, Peabody, Mass.) at an accelerating voltage of 25 kV. Samples weremounted on an aluminum stub and sputter coated with gold to minimizecharging. To obtain more information about the internal structure of thehollow microspheres, dry etched polymer particles were sheared betweentwo glass slides after freezing in liquid nitrogen to obtain crackedbeads using standard procedures. This technique allows determination ofthe polymer shell thickness.

Atomic Force Microscopy (AFM)

Surfaces of hollow polymer microspheres were imaged using a DigitalInstruments Nanoscope IIIa scanning probe microscope. (DigitalInstruments Inc., Santa Barbara, Calif., USA). Images were acquired inTapping mode with the Z range set at 4.0 μm. Scan size was 7.0 μm Thescan rate was 0.5 Hz. Images were acquired using a diamond coatedtapping mode tip (L=125 μm, F_(o)=360 kHz). Samples of hollow polymermicrospheres were allowed to dry from an aqueous suspension onto a glassmicroscope slide prior to AFM analysis. Surface roughness analysis wasperformed using Digital Instruments Nanoscope software (version 4.10).

Gel Permeation Chromatography (GPC)

Molecular weights and molecular distributions were obtained on a Waters2690 Separation Module (Waters Corporation, Milford, Mass.) connected toa Waters 410 Differential Refractometer with THF as the carrier solvent.Molecular weights were calibrated using polystyrene standards.

Characterization of Hollow Polymeric Microspheres Produced bySurface-Confined Living Radical Polymerization on Silica Templates

Spherical silica particles with an average diameter of 3μ were used as atemplate for the synthesis of uniform hollow poly(benzyl methacrylate)microspheres. The ((chloromethyl)-phenylethyl)trimethoxysilane (CTMS)initiator was attached to the silica surface by treating the silica withCTMS in acetone. Upon curing, a covalently linked benzyl chloridemonolayer is formed on the silica microsphere surface. Elementalanalysis results showed that the initial silica microparticles contained<0.02% chlorine and that the CTMS-attached microparticles contained3.15% chlorine (Galbraith Laboratories, Inc., Knoxville, Tenn.). Thisdifference is equivalent to an average of 0.88 mmol initiator/g ofsilica. The grafting density of the monolayer of benzyl chloride was 2.3μmol/m², calculated on the basis of average surface area (400 m²/g, datasupplied by Phenomenex) of the pure silica particles. The resultingsurface modified silica particles could be redispersed in organicsolvents. Scanning electron micrographs of the CTMS modified silicamicroparticles showed that they remain unaggregated (FIG. 1A) and weresimilar to the original silica microparticles, exhibiting nocharacteristic features. Although, benzyl chloride (—Ph—CH₂Cl) of CTMSis generally not an efficient initiating group for atom transfer radicalpolymerization compared to 1-phenylethyl chloride or bromide, itperformed adequately in this case. Silica microspheres are coated withhigher molecular weight PBzMA by using alternative initiators such asthose listed in Table 3.

The surface modified microparticles were then used as macroinitiatorsfor benzyl methacrylate atom transfer radical polymerization. Polymergrowth was confined to the surface of initiator-modified silicamicrospheres. The polymer coated silica microspheres were dispersedeasily in good solvents for poly(benzyl methacrylate)(PBzMA). FTIRspectra of the resulting composite particles showed bands correspondingto both poly(benzyl methacrylate) and silica. A SEM micrograph of thepolymer coated silica microspheres shows that the polymer is uniformlycoated over the silica surface (FIG. 1B). Tapping mode atomic forcemicroscopy was used to obtain more detailed information about thesurface topography. The AFM image of the surface of hollow polymermicrospheres shows that the surface was very smooth. The rootmean-square roughness (Rq) value is 8-10 nm. This value compares wellwith Rq values for silanized non-porous silica microspheres. ATRP formsprimarily monodisperse polymer chains, with a uniform surface coating.The thickness of the polymer layer increases with increasingpolymerization time at fixed monomer concentrations. Although thepossibility exists that when polymer chains are densely grafted to asurface, steric crowding forces the chains to stretch away from thesurface, the curvature of the silica particles may help to reduce stericcrowding. Overall, the thickness of the polymer layer should be largerthan the radius of gyration for the equivalent free polymer in solution.

Polymer/silica particle composites were converted to hollow polymericmicrospheres by immersing a pellet of the composite particles (supportedby a Fluopore membrane) in an aqueous solution of HF. Silica dissolutionoccurs via transport of etchant through the polymer shell to the core.FIG. 2A shows the SEM micrograph of the aggregated intact hollow PBzMAmicrospheres after etching the silica core. Aggregated hollow polymerparticles were redispersed as individual particles by sonicating aportion of the pellet in water (FIG. 2B). When the composite particlesare prepared by 1 h polymerization, no hollow microspheres are obtainedafter HF etching. This result indicates that the polymer shell thicknesswas not sufficient to maintain the initial spherical structure of thesilica microsphere upon core removal. The hollow microspheres aresoluble in THF and other organic solvents because the polymer chains areno longer grafted to the solid silica surface. This result proves thatthe silica cores are completely etched by the HF solution. Shellcross-linked hollow polymer microspheres, however, are not soluble inmost organic solvents. For this reason, they are useful for drugdelivery or encapsulating drugs/dyes in non-aqueous solvents.

After etching the silica core, the spheres were dissolved in THF and themolecular weight of the dissolved polymer was determined by GPC. Themolecular weights of three samples of cleaved surface initiated PBzMA,prepared with different polymerization times, are given in Table 4.

TABLE 4 Shell thicknesses and molecular weights of the hollowpoly(benzyl methacrylate) microspheres prepared by varyingpolymerization time Polymerization time Shell thickness Molecular weightPDI (hr) (nm) (M_(n)) (M_(w)/M_(n)) 3.5 175-225 9150 1.56 6.5 350-40013450 1.37 14.0 550-600 26500 1.26 Conditions: 6-7 mg of CTMS-modifiedsilica microparticles (0.88 mmol of initiator); [CuCl] = 90.6 mM; [bipy]= 280 mM; [benzyl methacrylate] = 7.57 M, and p-xylene solvent at105-110° C.

The molecular weight (Mn) of the grafted polymer, as determined by GPCincreased with polymerization time. The molecular weight distribution(Mw/Mn) remained narrow after the initial stage of polymerization. Thepolydispersity indices are consistent with that expected from livingpolymerization (PDI<1.5) for the 6.5 h and 14 h cleaved samples,although, the polydispersity of the 3.5 h sample is somewhat higher than1.5.

In order to confirm that the microspheres were hollow, they were frozenin liquid nitrogen and then crushed between two glass plates. FIG. 3shows a SEM image of intact and broken polymer microspheres. Brokenhollow PBzMA microspheres produced by varying the polymerization timeare shown in FIGS. 4A-C. The shell thicknesses were measured from theSEM micrograph of the broken hollow PBzMA particles, which are given inTable 4. The data reveal that shell thickness increases with increasingpolymerization time. Measured shell thicknesses of the samples preparedwith different polymerization times were higher than expected based onthe calculated values for the fully extended chains from theirrespective molecular weights. Higher shell thickness values may be dueto a number possibilities. First, shell thickness was measured using SEMafter the hollow polymer microspheres were freeze-fractured. It ispossible that measured shell thickness is artificially high due todistortion of the polymer since the microspheres were frozen andcompressed between glass plates prior to fracturing. A secondpossibility is the formation of polymer inside the pores of the silicatemplates. Polymer chain attachment and growth at different distancesfrom the template center contributes to the observed shell thicknessafter etching. The silica core dissolution process may also affect theshell thicknesses. For example, when HF diffuses through the polymershell and reaches the core, it reacts with silica to form silicontetrafluoride gas and the polymer chains detach from the surface attheir point of attachment. The resulting gas from the interior mayproduce micro voids inside the polymer shell and result in an increasedshell thickness. The detached polymer chains have no solid support, andmay also aid in the void generation.

For further confirmation that the polymer microspheres contain little orno silica inside the core, FTIR characterization was performed on etchedhollow PBzMA particles and was compared with pure poly(benzylmethacrylate) and neat silica. FIG. 5 shows the FTIR spectra of puresilica, PBzMA coated silica particles and PBzMA particles after etchingthe silica core. Spectra for polymer-coated silica particles (plot b)reveal bands at 750 and 697 cm⁻¹ corresponding to phenyl C—Hout-of-plane bending, and benzene out-of-plane ring bendingrespectively, and the 1728 cm⁻¹ carbonyl stretching vibrationscharacteristic of PBzMA. In addition to the PBzMA signals, a broadintense signal in the 1350-1000 cm⁻¹ region corresponds to the solidstate vibration of the Si—O—Si bond in silica. The FTIR spectrum of thehollow PBzMA particles (plot c) and cross-linked hollow particles (notshown) is similar to the spectrum of neat PBzMA (plot d) and shows nospectral characteristics of silica, confirming that the silica coreswere etched completely.

The data described herein demonstrate that the reliability andpredictability of a procedure for making uniform hollow microspheresusing surface-initiated controlled/living radical polymerization onsilica templates followed by core removal by etching. This method isflexible and enables control over the shell thickness and composition byadjusting polymerization time and monomer concentration. This approachis useful for the fabrication of different shapes of hollow polymericmaterials produced from a variety of templates. The method is alsouseful to produce hollow microspheres containing different polymerlayers by the sequential activation of the dormant chain in the presenceof different monomers during polymerization.

Nanosphere-Microsphere Assembly Methods for Preparing Core-ShellComposite Microsphere Compositions and Hollow Polymeric Microspheres

Colloidal assembly is a process by which particles ranging in size fromnanometers to micrometers are organized into structures by mixing two ormore particle types. Assembly is controlled by either specific ornon-specific interactions between particles. Examples include chemicalbonding, biological interactions, electrostatic interactions, capillaryaction and physical adsorption. The assembly process is performed suchthat smaller particles assemble around larger ones.

The colloidal assemby method described herein includes specific chemicaland biochemical interactions, which are manipulated to control particleassembly. Polymer nanospheres are assembled onto the surface of silicamicrospheres, and the assembled composite is subsequently heated to atemperature above the Tg of the polymer nanospheres allowing the polymerto flow over the silica microsphere surface, resulting in a uniformcore-shell composite. The methods used to assemble 100 and 200 nmdiameter amine-modified polystyrene (PS) nanospheres onto 3-10 μmdiameter glutaraldehyde-activated silica microspheres. SFM was used toestimate the packing density of polymer nanospheres on the silicamicrosphere surfaces. The biospecific interaction between avidin andbiotin was also used to control the assembly of PS nanospheres ontosilica microspheres. Avidin, a 40 kD glycoprotein, is known to have fourhigh affinity binding sites for the vitamin derivative biotin(MW=244.31). When avidin-labeled PS nanospheres were mixed withbiotin-labeled silica in the appropriate number ratio, PS nanospheresassembled onto the microsphere surfaces, covering the microspheres.Thermally annealed composites, produced by heating PS nanosphere-silicamicrosphere assemblies at temperatures higher than the Tg of PS, werecharacterized using several analytical techniques. The compositions ofthe resulting core-shell materials were confirmed by FTIR spectroscopyto be PS-silica composites. The uniformity of the shell material coatingwas confirmed by scanning electron microscopy (SEM). Core silicaparticles were etched with hydrofluoric acid to confirm the existence ofthe shell structure. The resulting hollow polymer microspheres werecharacterized by transmission electron microscopy (TEM). Compositepolymer shell core-shell materials were also produced by mixing PS andpoly (methylmethacrylate) nanospheres in varying ratios prior toassembly and annealing.

Uses for Core-Shell Composite Compositions

The shell material of a core-shell composite is used to allow dispersalof the core composition in a particular solvent or to protect the corefrom dissolution in the solvent. For example, core-shell materials areprepared with polymer shells to protect medicines or other materialsfrom dissolution or hydrolysis. Polymer shells are used to stabilizepigments in paints. Core-shell materials are also be useful tostrengthen polymeric materials. Other areas of application include thepreparation of stationary phases for chromatography or in thepreparation of sensing materials. For example, a thin polybutadiene filmcan be physically adsorbed onto zirconia surfaces and then cross-linked,resulting in a stationary phase for reversed-phase chromatography withexceptional stability at high pH. Core-shell nanoparticles loaded withgadolinium are useful as contrast agents for magnetic resonance imaging.

The following reagents and methods were used to construct microspheresusing colloidal assembly.

Reagents

Amine-labeled porous silica microspheres (˜3 and 5 mm diameter) wereobtained from Phenomenex Inc. (Torrance, Calif., USA). Amine-modifiedpolystyrene (PS) nanospheres (100 and 200 nm mean diameter) wereobtained from Polysciences Inc. (Warrington, Pa., USA). 25%glutaraldehyde solution (aqueous), ethanol and ethylene glycol wereobtained from Sigma-Aldrich Chemical Co. (Milwaukee, Wis., USA), Avidin(neutravidin) and biotin were obtained from Molecular Probes Inc.(Eugene, Oreg., USA). N-hydroxysuccinimide (NHS),1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) andmorpholinoethanesulfonic acid (MES) were obtained from Pierce ChemicalCo. (Rockford, Ill., USA). All solutions were prepared using ultra-purewater (Bamstead/Thermolyne, Dubuque, Iowa., USA).

Nanosphere-Microsphere Preparation

Amine-labeled silica microspheres were prepared for assembly by firstactivating with glutaraldehyde. Prior to activation, approximately 2-6mg of dry microspheres were placed into an eppendorf tube and washedfive times with 1.0 mL of ultra-pure water. Microspheres were cleanedusing five cycles of centrifugation, supernatant removal andresuspension in 1.0 mL of ultra-pure water. Microspheres werecentrifuged at 5,000×g. The microspheres were then washed two times with1.0 mL of 50 mM phosphate buffer pH 6.9. The microspheres were thencentrifuged again (at 5,000×g), the supernatant removed, and 1.0 mL of a2.5% glutaraldehyde solution in 50 mM phosphate buffer pH 6.9 was added.The microspheres were suspended, covered with foil and mixed on a vortexshaker for two hours at 4° C. After two hours the microspheres werewashed five times with 1.0 mL of ultra-pure water. Microspheres werewashed another five times to exchange them into a 50 mM phosphate bufferpH 7.4. The microspheres were stored at 4° C. protected from light untilneeded for the assembly process. Amine-modified polystyrene nanosphereswere similarly prepared for assembly by washing (centrifuging at18,000×g and resuspending in water) five times with 1.0 mL of ultra-purewater and two times with 1.0 mL of 50 mM phosphate buffer pH 7.4.Nanospheres were stored at 4° C. until they are used in the assemblyprocess. 100 mL of a 2.7% (w/v) suspension of nanospheres are routinelyprepared using this procedure.

Biotin-labeled silica microspheres were prepared by treating 2-4 mg ofamine-labeled silica microspheres with 1.0 mL of a 5 mM solution ofbiotin-SE in 0.13 M sodium bicarbonate buffer pH 8.3. The microsphereswere suspended and shaken on a vortex shaker for one hour at 4° C.Excess biotin-SE is removed with several cycles of centrifugation(5,000×g), supernatant removal and resuspension with 1.0 mL of 50 mMphosphate buffer pH 7.4. The microspheres were stored at 4° C. untilused in the colloidal assembly process. Alternatively, silicamicrospheres were labeled with avidin by treating 2-4 mg ofglutaraldehyde-activated silica microspheres with 1.0 mL of a 2 mg/mLavidin in phosphate buffer pH 6.9 for 2 hours at 4° C. Excess avidin wasremoved by several cycles of centrifugation and resuspension in 50 mMphosphate buffer pH 7.4.

Biotin labeled nanospheres were prepared as follows: 100 mL of a 2.7%(w/v) suspension of amine-modified polystyrene nanospheres was washedfour times with 1.0 mL of ultra-pure water then with 1.0 mL of 0.13 Msodium bicarbonate buffer pH 8.3. Biotin-SE was then added to a finalconcentration of 5 mM. The nanospheres were shaken on a vortex mixer forone hour at 4° C. Subsequently excess biotin-SE was removed with threecycles of centrifugation/resuspension in 1.0 mL of 50 mM phosphatebuffer pH 7.4. When avidin-modified nanospheres were needed for theassembly process, biotin-modified nanospheres (in 50 mM phosphate bufferpH 7.4) were treated with 1.0 mL of a 0.1 mg/mL solution of avidin in 50mM phosphate buffer pH 7.4, The nanosphere/avidin suspension was gentlymixed and then shaken for two hours at 4° C. on a vortex shaker.Subsequently, the nanospheres were washed seven times with 1.0 mL of 50mM phosphate buffer pH 7.4. Avidin-modified nanospheres were stored at4° C. until they were used in the assembly process.

Poly(Methyl Methacrylate)PMMA Nanosphere Preparation

Amine-modified PMMA nanospheres were prepared from carboxyl-modifiedPMMA nanospheres by conversion of the carboxyl groups to a succinimidylester and then treating the nanospheres with ethylenediamine. Theprocedure was as follows: 100 mL of a 2.7% (w/v) suspension of PMMAnanospheres (80 nm mean diameter) was washed five times with ultra-purewater, then two times with 50 mM MES pH 4.75 containing 0.5% (w/v) NaCl.Next 1.0 mL of a 10 mM NHS/60 mM EDC solution in MES buffer pH 4.75 wasadded and the nanospheres were suspended and mixed on a vortex mixer onlow setting. Mixing continued for one hour at 4° C. in the dark. Afterone hour, the nanospheres were centrifuged (18,000×g) and thesupernatant removed. One mL of fresh NHS/EDC solution was then added andthe nanospheres were suspended and mixed. Nanospheres were shaken at 4°C. for another hour. After this time, the nanospheres were immediatelycentrifuged (18,000×g, 15 minutes), the supernatant was removed and 1.0mL of a 10 mM ethylenediamine solution in 50 mM phosphate buffer pH 7.4was added. The nanospheres were suspended and mixed on a vortex shaker.The reaction was allowed to continue at 4° C. for one hour. Followingtreatment with ethylenediamine, the nanospheres were washed two timeswith ultra-pure water and then five times with 50 mM phosphate buffer pH7.4. Amine-modified PMMA nanospheres were stored at 4° C. until used inthe assembly process

Nanosphere-Microsphere Assembly

The colloidal assembly process described herein was controlled by eitherspecific chemical or biochemical interactions. The reactions ofamine-modified polystyrene nanospheres with glutaraldehyde-activatedsilica microspheres and avidin/biotin labeled polystyrene nanosphereswith avidin/biotin silica microspheres were used to direct the assembly.Amine-modified PS nanospheres were assembled onto aldehyde-activatedsilica microspheres as follows: Aldehyde-activated silica microspheres(2-4 mg) were suspended in 50 mM phosphate buffer pH 7.4. Depending onthe particle sizes; this suspension contained microsphere concentrationof approximately 6.0×10⁸ particles/mL. An appropriate volume of asuspension of amine-modified PS nanospheres in phosphate buffer pH 7.4was then added so that a 5000:1 number ratio of nanospheres tomicrospheres was achieved. The suspension was shaken at 4° C. for 12-18hours on a vortex mixer. Subsequently the product was purified byalternately centrifuging (200×g) and resuspending the assembled productin ultra-pure water. An identical process was followed when assembly wascontrolled by the biospecific interaction of avidin and biotin labelednanospheres and microspheres.

Nanosphere-Microsphere Assembly Melting

In order to produce a material with a core-shell morphology thenanosphere-microsphere assemblies were heated at 170-180° C. in ethyleneglycol using a temperature-controlled hot plate with a silicone oilbath. As the temperature increased above the glass transition (Tg) ofthe polymer nanospheres, the polymer melted and then flowed over thesurface of the silica microsphere templates. As a result, uniformcore-shell materials consisting of a silica core and polymer shell wereproduced. In order to prepare the nanosphere-microsphere assemblies formelting two milligrams of the assembled product was suspended in 250 mLof ethylene glycol.

Ethylene glycol was chosen as the solvent because it has a high boilingpoint and polystyrene is insoluble in it. This suspension was then addedto 750 mL of ethylene glycol in a glass vial maintained at 170-180° C.(silicone oil bath). The mixture was stirred vigorously for 5-10minutes. The mixture was then removed from the oil bath and sonicatedwhile cooling in room temperature water. The suspension was centrifugedand resuspended in ethanol two times. The suspension was dried on apiece of aluminum and subsequently was brought to a temperature of170-180° C. The composite was heated at this temperature for 20 minutes.After this time the product was allowed to cool, removed from the metaland resuspended in ultra-pure water. The product was sonicated for twominutes, then centrifuged and resuspended in ultra-pure water anadditional two times. To melt the avidin-biotin directed assembly, thecomposite was first washed with ethanol then applied in a thin layer onan aluminum metal block. The block was then heated at 170-180° C. for20-30 minutes to allow the assembled PS nanoparticles to melt and flowover the silica microsphere surfaces.

Electron Microscopy

SEM and TEM analysis was performed using standard techniques andinstrumentation.

Fourier Transform Infra Red (FTIR) Spectroscopy

FTIR (Nicolet Magna-760, Nicolet Instrument Corporation, Madison, Wis.)spectroscopy was used to identify the polymer on the microspheresurfaces. Spectra were obtained at a resolution of 2 cm⁻¹ and averagesof 64-100 spectral/scans (for enhanced signal) were obtained in thewavenumber range 400˜4000 cm⁻¹. All samples were prepared for analysisusing a KBr pellet. Pellets were prepared using a 50:1 weight ratio ofKBr to sample. All spectra were acquired at room temperature.

Scanning Force Microscopy

Surfaces of composite microspheres were imaged using a DigitalInstruments Nanoscope IIIa scanning probe microscope. (DigitalInstruments Inc., Santa Barbara, Calif., USA). Images were acquired inTapping mode under standard conditions.

Nitrogen Analysis

Weight percent nitrogen was determined by Galbraith Laboratories Inc.(Knoxville, Tenn., USA) Oven-dried samples of amine-labeled silicamicrospheres and amine-modified polymer nanospheres were submitted foranalysis.

Chemical Etching of Core-Shell Composites

The silica cores were etched using an 8% aqueous solution ofhydrofluoric acid. Approximately 1 mg of polystrene-silica composite wassuspended in 1.0 mL of ultra-pure water. Concentrated hydrofluoric acid(50% w/v) was then added to bring the total HF concentration to 8%. Thesuspension was allowed to stand for 20 minutes to assure completeremoval of the silica cores. The composite was then washed five timeswith 1.0 mL of ultra-pure water. The resulting hollow polymermicrospheres were then air dried on glass slides prior to SEM or TEManalysis.

Characterization of Microspheres After Colloidal Assembly

The general procedure for the colloidal assembly of polymer nanosphereswith silica microspheres is shown in FIG. 12.

The assembly process was performed by mixing a suspension ofcomplementary types of nanospheres and microspheres at 4° C. for 12-18hours. The assembly process was designed to pack as many nanospheresonto the microsphere surface as possible. The numbers of nanospheres tobe packed on the microsphere surfaces was calculated by dividing thetheoretical microsphere surface area by the cross-sectional area of aplane bisecting a nanosphere. The resulting value was used to determinethe minimum number of nanospheres needed in suspension for eachmicrosphere present. The number of nanospheres required to completelycover a microsphere can be calculated using known methods, e.g.,Ottewill et al.,1997, Colloid Polym. Sci. 275: 274-283. The calculationis based on hexagonal close packing of the nanospheres onto a planarsurface. Following assembly, the composites were heated at 170-180° C.to melt the polymer nanospheres (FIG. 12).

After heating, the polymer melts and flows over the microsphere surfacesto yield uniform core-shell materials consisting of a silica core and apolymer shell. Representative SEM images of these materials are shown inFIGS. 13A-D. An SEM image of 100 nm amine-modified PS particlesassembled onto 3 μm diameter glutaraldehyde-activated silicamicrospheres is shown in FIG. 1A. FIG. 13B is an SEM of 200 nm diameteramine-modified PS particles assembled onto 5 μm diameter silicamicrospheres. An SEM of 100 nm avidin-labeled nanospheres assembled onto3 μm diameter biotin-labeled silica microspheres is shown in FIG. 13C.Tapping mode SFM was used to image the surfaces of compositemicrospheres (5 μm diameter) that were assembled with 200 nm PSnanospheres. The SFM scan shows that the 200 nm PS particles areassembled in a dense array on the surface of the microspheres (FIG. 14).

The packing density of the polymer nanospheres on the surface of thesilica microspheres is an important variable in the formation of auniform polymer shell around the silica microspheres. Packing densitywas easily and reliably controlled when the assembly process wascontrolled by amine-aldehyde chemistry. The packing density was slightlymore variable when assembly was controlled by the interactions of avidinand biotin-labeled colloidal particles. In some cases, aggregation mayarise during the synthesis of the avidin-labeled nanospheres, becausethe nanospheres are first labeled with biotin and then subsequentlytreated with an excess of avidin. Cross-linking of nanospheres may occurif the concentration of nanospheres in the suspension is too highrelative to the amount of avidin used. Aggregation may also occur wheninsufficient numbers of biotin-labeled nanospheres are mixed withavidin-labeled silica microspheres. Despite these factors associatedwith the use of avidin-biotin, the assembled composites are comparablein uniformity of thickness to those formed when amine-glutaraldehyde isused to control the assembly process.

Since the methods used to control the assembly process involve specificchemical and biochemical interactions, it was necessary to verify thatthe assembled composites were the result of these specific interactionsbetween the particles and not to non-specific interactions. Non-specificbinding during the assembly process was minimal for both theamine-glutaraldehyde and avidin-biotin methods. Percentages ofnon-specific binding were estimated based on the theoretical maximumnumber of nanospheres that could cover one-half of a microspheresurface. The number of nanospheres visible in SEM images of controlsassembled with non-specific binding were counted and taken as thepercentage of the theoretical maximum. Approximately 10-12% non-specificbinding was observed when amine-modified nanospheres were mixed withamine-coated silica microspheres. The weight % nitrogen was 0.73% and<0.5% for the amine-labeled silica microspheres and amine-modifiedpolystyrene nanospheres respectively. An SEM image of microspheresprepared using non-specific binding (control) is shown in FIG. 15.

Other non-specific binding controls included mixing unmodifiedpolystyrene nanospheres with aldehyde-activated silica microspheres,amine-modified polystyrene nanospheres with unmodified silicamicrospheres and unmodified polystyrene nanospheres with unmodifiedsilica microspheres. In each of these cases, non-specific binding was<1%. A non-specific binding control for avidin-biotin directed assemblywas performed by mixing avidin-labeled nanospheres with avidin-labeledsilica microspheres. Approximately 1% non-specific binding was observed.

The assembled composites prepared by either the amine-glutaraldehyde oravidin-biotin methods were very stable (as observed by SEM). Nonoticeable changes in the surfaces of the materials were observed uponseveral weeks storage in solution at room temperature or at 4-8° C.Suspension in ethanol or ethylene glycol had no effect unless thetemperature was increased above the glass transition (Tg) of PS.Stability of the assembled composites in ethylene glycol was importantsince the melting procedure was performed at elevated temperature inthis solvent.

The polystyrene nanosphere/silica microsphere assemblies were heated inethylene glycol under the premise that the polymer nanospheres wouldmelt and the polymer would flow over the silica microsphere surfaces,producing a core-shell composite with a uniform polymer coating.Ethylene glycol was chosen as the solvent for heating the materialsbecause it has a high boiling point and because polystyrene and manyother polymers are insoluble in it. Microsphere aggregation during theheat treatment was minimized by controlling the concentrations ofmicrospheres in solution. Annealing the composites on an aluminum metalblock after the initial heating in ethylene glycol helped to improve theuniformity of the polymer coating. Melting of the 100 nm PS/3 μm silicamicrosphere assembly (FIG. 3.3 a) at high temperature in ethyleneglycol, followed by heating on an aluminum block results in the uniformPS-silica core-shell composite shown in FIG. 13D. Avidin-biotinassembled composites could only be melted on an aluminum metal surfacebecause melting in an ethylene glycol solution did not result inuniformly coated core-shell composites. This result may be due to theinstability of the avidin-biotin linkage in solution at the hightemperatures used to melt the nanospheres, which causes them todissociate.

To verify that polystyrene was coating the silica microspheres, twoindependent evaluations were performed. A time study was conducted byheating the polystyrene nanoparticle-coated silica microspheres inethylene glycol and then removing aliquots of assembled microspheres atvarious times during the course of the 30-minute heating. SEM imagesshowed that nanoparticles remained attached to the silica microspheresurface after five minutes. As the heating time increased, the sphericalnanoparticles melted and gradually filled in the spaces betweennanoparticles until the surface was uniformly coated. The integrity ofthe polymer shells was determined by removing the silica cores bychemical etching with hydrofluoric acid. After chemical etching, hollowpolymer shells were all that remained of the composite. The hollowpolymer microspheres produced remained intact after sonication inultra-pure water and centrifugation at 2,000×g. This result providesadditional evidence of polymer coating, since the silica microspheres donot survive such treatment. A TEM of hollow polymer shells produced byassembling 200 nm PS nanospheres onto 3 μm silica microspheres, followedby annealing and chemical etching is shown in FIG. 16.

FTIR spectra (FIG. 17) of the PS-silica core-shell composites provideadditional evidence that the melting procedure results in polymer coatedmicrospheres. The methods described herein provide considerable controlin the assembly process to consistently yield core-shell compositionsand hollow micropheres in which the thickness of the shell isessentially uniform, i.e., the thickness varies less than 10%.

The spectra reveal bands at 750 cm⁻¹ and 697 cm⁻¹, which correspond tothe phenyl C—H out-of-plane bending and benzene out-of-plane ringbending respectively. Both of these resonances are characteristic ofpolystyrene and are absent from the starting silica microspheres.Aliphatic C—H stretching resonances of polystyrene (2900 cm⁻¹) can beseen in the FTIR spectra shown in FIG. 18.

A 50:50 mix of PS and poly (methylmethacrylate) nanospheres resulted inan assembled composite with a polymer nanosphere compositioncorresponding to this ratio. This assembly was heated at 170-180° C. inethylene glycol to melt both nanosphere types assembled on the silicamicrospheres. The resulting core-shell composite was confirmed by FTIRspectroscopy to be a PS-PMMA composite (FIG. 19), The data describedherein indicate that assembled materials predictably produce core-shellcomposites, e.g., those containing a silica core and a polystyrene shellof essentially uniform thickness. The methods can be used to create ashell that is a composite of multiple polymer types by mixing polymernanospheres in the ratio desired prior to assembly. Such materials haveapplications in both analytical and materials chemistry development.Core-shell composite materials are useful in the design of layeredsensing materials, the production of stationary phases forchromatographic separations or the development of drug delivery systems.

Nanosphere/microsphere assembly accesses novel materials a reliable andflexible procedure. By selecting the compositions of the particles usedin the assembly procedure, considerable control is gained over thephysical and chemical properties of the resulting composites. Additionalcontrol over physical/chemical properties is achieved by the ability tomelt assembled polymer particles yielding uniform silica core/polymershell composite materials. The use of specific chemical/biochemicalinteractions to control the assembly process of colloidal particles hasseveral advantages over the use of electrostatic interactions orheterocoagulation to prepare core-shell composites. One advantage isthat a wider range of materials may be assembled when specificinteractions are used. For example, particles that are not charged orhave the same charge are assembled using this technique. Amine-modifiedPS nanospheres are assembled onto amine-labeled silica by activating thesilica surface with a cross-linking dialdehyde. Another advantage is theimproved stability of the assembled products when covalent or strongbiospecific interactions are employed. The stability of the bondsbetween the particles allows the use of a wider range of pH's, ionicstrengths and solvents in the assembly process.

Other embodiments are within the following claims.

1. A hollow microsphere comprising a polymeric shell, wherein thethickness of said shell varies less than 10%.
 2. The microsphere ofclaim 1, wherein said shell thickness varies less than 5%.
 3. Themicrosphere of claim 1, wherein said shell thickness varies less than1%.
 4. The microsphere of claim 1, wherein said shell thickness variesless than 0.5%.
 5. The microsphere of claim 1, wherein said shellthickness is in the range of 100-1000 nm.
 6. The microsphere of claim 1,wherein said shell thickness is in the range of 150-250 nm.
 7. Themicrosphere of claim 1, wherein said shell thickness is in the range of350-450 nm.
 8. The microsphere of claim 1, wherein said shell thicknessis in the range of 550-650 m.
 9. The microsphere of claim 1, whereinsaid microsphere is substantially devoid of silica.
 10. The microsphereof claim 1, wherein said microsphere comprises a pore, said pore havinga size in the range of 10 -500 nm.
 11. The microsphere of claim 1,wherein said microsphere comprises an organic dye.
 12. The microsphereof claim 11, wherein said dye is an Azo dye.
 13. The microsphere ofclaim 11, wherein said dye is selected from the group consisting ofIndigo Blue, Lissamine Green B, VAT Green 1, VAT Yellow 4, VAT Violet 1,Anthrasol, Blue IBC, Indigosol Pink IR, Indigosol Grey IBL, AnthrasolBrown IBR, and Red
 146. 14. The microsphere of claim 1, wherein saidmicrosphere comprises a protecting agent.
 15. The microsphere of claim1, wherein said microsphere comprises a therapeutic agent.
 16. Themicrosphere of claim 15, wherein said therapeutic agent is selected fromthe group consisting of a polypeptide, an antibody, an enzyme, a nucleicacid, and a small molecule drug.
 17. The microsphere of claim 1, whereinsaid shell comprises an acrylate polymer, a methacrylate polymer, or astyrene polymer.
 18. The microsphere of claim 1, wherein said shellcomprises a polymer of one or more monomers selected from the groupconsisting of acrylonitrile, styrene, benzyl methacrylate, phenylmethacrylate, ethyl methacrylate, divinyl benzene, 2-Hydroxyethylmethacrylate, cyclohexyl methacrylate, p-methyl styrene, acrylamide,methacrylamide, methacrylonitrile, hydroxypropyl methacrylate, methoystyrene, N-acrylylglycinamide, and N-methacrylylglycinamide.
 19. Themicrosphere of claim 1, wherein said shell comprises a co-polymerselected from the group consisting of styrene-PMMA, benzylmethacrylate-PMMA, styrene-PHEMA, styrene-PEMA, styrene-methacrylate,and styrene-butylacrylate.
 20. The microsphere of claim 1, wherein saidpolymeric shell comprises a cross-linked polymer.
 21. The microsphere ofclaim l, wherein the microsphere is prepared by a method comprising:providing a substrate comprising a plurality of hydroxyl groups;attaching an initiator agent to said hydroxyl groups to form attachedinitiator agents; reacting the attached initiator agents with apolymerizable unit under living polymerization conditions to form apolymer shell over said substrate, said polymerization being confined toa surface of said substrate; and exposing said substrate to an etchingagent for a time sufficient to allow for removal of said substrate fromsaid polymeric shell to form a hollow microsphere.
 22. The microsphereof claim 1, wherein the microsphere is prepared by a method comprising:providing a microsphere substrate; contacting said microsphere substratewith a polymer nanosphere to yield a colloidal assembly; heating saidassembly to yield a core-shell composite; and exposing said composite toan etching agent for a time sufficient to allow for removal of said corefrom said shell to form a hollow microsphere.